This invention relates to intermittent coating techniques using an extrusion coater, and more particularly to an intermittent coating method adapted to intermittently apply coating liquid to a web continuously traveling to alternately form the web with coated regions which are deposited thereon with the coating liquid and uncoated regions which are not deposited thereon with the coating liquid and an apparatus therefor. More specifically, the present invention relates to an intermittent coating method used in manufacturing of a lithium ion secondary cell and an apparatus therefor.
An electrode which has been conventionally used for a lithium ion secondary cell of a cylindrical shape is typically constructed in such a manner as shown in FIG. 9. More particularly, the electrode includes a current collector or conductive sheet 1 which is formed on each of both surfaces thereof with a coated film 2 containing an active material while being partially exposed. Then, the conductive sheet 1 is provided on a portion thereof thus exposed with a lead 3. For manufacturing of such an electrode, typically, a lengthy conductive sheet material or web is intermittently coated on one surface thereof with a coating liquid containing an active material, so that coated regions which are deposited thereon with the coating liquid and uncoated regions which are not deposited thereon with the coating liquid may be alternately formed on the conductive sheet material, followed by drying of the conductive sheet material. Substantially the same procedure is repeated on the other surface of the conductive sheet material and then it is cut into a plurality of the above-described conductive sheets 1. Then, the lead 3 is mounted on each of the conductive sheets 1, resulting in the electrode being provided.
Intermittent coating or application of the coating liquid onto the conductive sheet material has been conventionally carried out using techniques disclosed in Japanese Patent Application Laid-Open Publication No. 184069/1989 or Japanese Patent Application Laid-Open Publication No. 226173/1994. More particularly, Japanese Patent Application Laid-Open Publication No. 184069/1989 discloses coating techniques wherein feeding of coating liquid from a coating liquid retention section constructed of a doctor blade and a support member to an aluminum foil is controlled by a shutter member and then coated on the aluminum foil by means of the doctor blade. Japanese Patent Application Laid-Open Publication No. 226173/1994 discloses coating techniques wherein coating liquid stored in a hopper is adhered to a coating roll while being controlled by a doctor roll and then the coating liquid adhered to the coating roll is applied to a base sheet carried on a backing roll.
Also, there is known Japanese Patent Application Laid-Open Publication No. 117669/1987 as other prior art, which discloses coating techniques wherein a bypass system is arranged for returning coating liquid discharged from a feed pump. A coating head is filled therein with coating liquid prior to application of the coating liquid and a directional control valve is changed over at the time when the application is started, resulting in the coating liquid being fed to the coating head.
Unfortunately, the coating techniques disclosed in Japanese Patent Application Laid-Open Publication No. 184069/1989 have a disadvantage of causing the coating liquid to leak between a lower surface of the shutter member and the support member during closing operation of the shutter member and failing to satisfactorily cut or shut off the coating liquid at an end position of a coated region, leading to undesired feeding of the coating liquid to an uncoated region beyond the end position of the coated region.
Also, the intermittent coating techniques disclosed in Japanese Patent Application Laid-Open Publication No. 226173/1994 cause forming of a stripe-like pattern in a coated film and/or a variation in thickness of the coated film due to coating liquid adhered to a portion of the doctor roll beyond an edge of the doctor roll, because the doctor roll is vertically displaced or the doctor roll is selectively moved toward or away from the coating roll. Forming of such a stripe-like pattern or a variation in thickness of the coated film may be eliminated by washing the doctor roll. However, this requires interruption of running or operation of a coating apparatus, leading to a deterioration in operation efficiency.
The coating techniques disclosed in each of Japanese Patent Application Laid-Open Publication Nos. 184069/1989 and 226173/1994 exhibit another problem that the coating liquid is varied in viscosity particularly when it is a solid-liquid dispersion paint containing an active material and/or a conductive material because it is exposed to an ambient atmosphere prior to application thereof, leading to a variation in coating properties of the coating liquid, resulting in a thickness of a coated film being rendered non-uniform.
Further, it was found that the above-described coating techniques disclosed in Japanese Patent Application Laid-Open Publication No. 117669/1987 have a disadvantage that application of the coating techniques to intermittent coating fails to permit the coating liquid to be satisfactorily cut or shut off at an end position of a coated region, leading to undesired feeding of the coating liquid to an uncoated region as well, resulting in a coated film formed being deteriorated in quality as in Japanese Patent Application Laid-Open Publication 184069/1989 described above. Thus, application of the coating techniques to intermittent coating fails to permit changing-over of the directional control valve at the end position of the coated region to reduce a pressure of coating liquid in a slit to a predetermined level, to thereby cause continuous feeding of the coating liquid until the pressure is reduced to the level.